Beverages, such as carbonated beverages, are typically packaged in cans made of metal, such as aluminum. Two piece cans are typically formed by seaming a can end to a can body. Traditionally, seaming is accomplished by forming a can end 10, shown in FIG. 1, in a die press. The can end 10 typically has a circular countersink bead 16, a substantially flat center panel 18, a seaming panel 13 that terminates in a peripheral curl 12, and a frustoconical side wall portion 14 that extends between the bead and the seaming panel. Traditionally, the side wall 14 is disposed at an angle A of about 14.degree. with respect to a line parallel to the centerline 7 of the can body 20. (Unless otherwise indicated, the numerical value of all angles referred to herein should be understood to be positive, meaning that the angle tapers away from the centerline of the can body as it extends upward in the direction from the bottom of the can body toward the can end. A negative angle is an angle that extends toward the centerline as it extends upward in the direction from the bottom of the can body toward the can end.)
Seaming is performed by disposing a flange 11 of the can body 20 under the seaming panel 13 on the can end 10. A seaming chuck 2 is then inserted into the can end 10, as shown in FIG. 1. Traditionally, seaming chucks 2 have frustoconical upper and lower wall portions 4 and 6, respectively. The lower wall portion 6 is typically disposed at an angle B that is a few degrees less than the angle A of the can end side wall 14 so that if the angle A of the can end side wall were about 14.degree., the angle B of the chuck lower wall would be about 11.degree.. The upper wall portion 4, which typically has a length L of about 0.130 inch, is typically disposed at an angle C that is about 4.degree.. Thus, the upper and lower wall portions 4 and 6 intersect at an edge 5 so as to form an obtuse angle of about 173.degree. (i.e., 180.degree. +4.degree. -11.degree.). Typically, the edge 5 has a radius of curvature of about 0.005 inch. Since the angle B of the lower wall 6 of the chuck 2 is less than the angle A of the can end lower wall 14, a relatively large gap, which may be as much as 0.010 inch, is formed between the chuck side wall and the can end side wall in the vicinity of the chuck wall edge 5, as shown in FIG. 1.
Seaming is completed by sequentially applying first and second seaming rolls against the curl 12 so as to press the curl and the flange 11 against the upper chuck wall 4, thereby producing a standard double seam 22, shown in FIG. 2.
Unfortunately, although pressed against the chuck 2 during seaming, the side wall 14 of the can end tends to spring back--that is, radially outward--when the pressure of the seaming roll is relieved. Thus, despite the fact that the upper and lower walls 4 and 6 of conventional seaming chucks 2 form two straight, frustoconical sections, the resulting side wall 14' of the can end 10' after seaming is arcuate, having a relatively large radius of curvature R.sub.1, as shown in FIG. 2. The curved nature of the seamed side wall 14' weakens the strength of the seamed can end 10'.
Recently, a non-standard can end has been developed in which the side wall, after seaming, is formed by two straight sections intersecting at a circumferentially extending crease. Such a can end is shown in published PCT application WO 96/37414. This structure is achieved by initially forming the can end side wall at a large angle that is said to be preferably in the range of 40.degree. to 45.degree.. According to the approach described in this published PCT application, the seaming chuck has a lower wall disposed at a similarly large angle and an upper wall disposed at an angle in the range of +4.degree. to -4.degree.. While this approach results in a strengthened can end, unfortunately, the large can end side wall angle required in this approach precludes its application to standard can ends, in which the side wall angle is only about 14.degree., as previously discussed.
Consequently, it would be desirable to provide a method and apparatus for seaming a conventional end to a can such that the resulting seamed can end had a side wall of improved strength.